How to install a heat pump into an existing property
Posted by John Barker-Brown on 21 February 2011 at 9:18 am
The advantages of installing a heat pump in a new build property is well documented. It is also possible to install heat pumps into existing properties, but there are various considerations which which determine how well, and even if, a heat pump will work effectively.
1. Sizing: the heat loss of the property is determined by how well the building is insulated. If the insulation is not known then it becomes difficult to size the heat pump accurately. The only way this can be achieved with any degree of accuracy is to commission a Standard Assessment Procedure report (SAP) which takes into account the build, insulation, heating system, etc and can be used to determine a peak heat load.
2. Insulation: This plays a big part in how effectively a heat pump operates. As the heat pump produces a lower flow temperature than traditional fossil fuel systems, the amount of heat it transfers to a room is limited (for a given heat emitter area) and is less than the traditional boiler.
If you raise the heat pump outlet temperature to its maximum (approx 50 degrees C) the heat pump has to work harder and its efficiency decreases. The coefficient of performance (CoP) at 50C is now only 3 as opposed to 4 (at 35C).
You also need to think about whether the actual heat emitting device, i.e. radiators, underfloor, etc, will output enough heat at the heat pump’s lower flow temperatures, to keep the building warm enough to be comfortable, particularly when the temperature outside is cold.
Reducing the energy requirement for any building should be a central theme to the design process. Any investment in an upgraded insulation specification will have a far swifter payback than the return on any renewable technology. For this reason, you should consider improving the insulation level beyond the minimum requirements stipulated in the Building Regulations.
If your building is very poorly insulated, the low temperature output from the heat pump may mean that it will never get warm and that the running costs for the heat pump are expensive. Possibly even higher than the existing fuel choice, particular if it is mains gas. We do not recommend installing a heat pump in a poorly insulated building.
3. The existing heating system: many existing properties will be heated by radiators. As radiators have a smaller surface area than underfloor heating, they need a higher flow temperature to provide heat. Before a radiator will provide any heat the flow temperature needs to be at least 45oC. This means that the efficiency of the heat pump is reduced and the Coefficient of Performance (CoP) drops from 4 (for underfloor heating in screed) to 3. This is a drop in efficiency of approximately 25% (and will result in higher electricity bills).
Due to the lower flow temperatures produced by a heat pump compared to a traditional fossil fuel boiler, the surface area of the radiator may need to be increased to provide the required heat into the building. Therefore all radiators will need to be oversized.
However, there are a number of situations which suit a radiator system, such as Social Housing schemes and clients may well accept the lower efficiencies of a radiator system for the ease of installation benefits, quicker response than underfloor heating, etc.
4. Land Available: This criteria only applies for ground source heat pumps, which uses renewable stored solar energy from the ground to heat the property; it absorbs this energy by means of a ground array buried within the property’s boundaries.
It is important to make sure that the correct amount of pipe for the application is buried and that it is buried correctly. If not enough pipe is installed then the ground could potentially run out of energy mid heating season, leaving you cold and without heat.
It is important to remember that if the heat pump is producing domestic hot water as well as space heating, more ground array is needed, as there is an additional all year round load on the ground. As a guide roughly 2 to 2.5 times the area being heated is required to install horizontal ground arrays.If you don't have enough land available an alternative to horizontally laid ground arrays is a vertical drilled borehole. These can be down to a depth of over 100m. Drilling a borehole is a specialist activity and as such can be expensive. As a guide these costs can be £40-£45 per m.
Any existing building where a heat pump is proposed needs to be looked at very carefully as a heat pump is not always the correct choice.
About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.
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